1. Field of the Invention
The present invention relates to a palladium activated sintering process to obtain glass-free refractory metal layers which are firmly secured to a ceramic substrate and which have a top surface that is substantially free of glass.
2. Description of the Prior Art
A relatively recent innovation in electronic packaging has been the development of the multilayer ceramic (hereafter MLC) module. In this technology "green" sheets of ceramic powder held together by a temporary organic binder are metallized with a noble or refractory metal, usually, but not mandatorily, by screen printing. The metallized sheets are stacked, laminated and fired to form a monolithic ceramic-metal package. Details on MLC technology are given in SOLID STATE TECHNOLOGY, May 1972, Vol. 15, No. 5, pages 35-40, Kaiser et al, hereby incorporated by reference. IBM Technical Disclosure Bulletin Vol. 17 No. 8 January 1975 p. 2331 and IBM Technical Disclosure Bulletin Vol. 12 No. 11 April 1970 p. 1795 disclose additional embodiments of related metallurgy for use on ceramic substrates.
Difficulty has been experienced in forming crackfree firmly adherent nickel or other solderable layers on a sintered refractory metal pad which has been subjected to a sintering cycle as above.
U.S. Pat. No. 3,218,194 Maissel discloses a method of stabilizing refractory metal film resistors wherein an oxidation resistant metal including palladium is applied to a refractory metal and the combination is heated in vacuum at an elevated temperature until the oxidation resistant metal film has completely diffused into and substantially filled the grain boundaries of the refractory metal film.
U.S. Pat. No. 3,518,756 Bennett et al discloses a method for forming multilayer ceramic structures wherein an electrode paste which may contain molybdenum and palladium is deposited by a silk screening step to form a desired circuit pattern.
U.S. Pat. No. 3,978,248 Usami discloses a method for manufacturing composite sintered structures wherein polyvinyl butyral is used. Molybdenum is disclosed as a useful conductor powder.
U.S. Pat. No. 3,999,004 Chirino et al discloses a multilayer ceramic substrate structure wherein blind holes in the structure are filled with a diluted metal paste which is about 83% molybdenum.
U.S. Pat. No. 4,109,377 Blazick et al discloses a method for preparing a multilayer ceramic wherein a useful metallizing composition includes molybdenum and a noble metal that can form an oxide, e.g., palladium.
In U.S. Pat. No. 4,340,618 Fury and Kumar, a method is disclosed for obtaining densely sintered refractory metal features free of glass by evaporating or sputtering a thin layer of palladium, etc., on a ceramic substrate prior to densification by sintering. Problems encountered with this method are: the deposition of palladium directly on the surface of the green ceramic blank in a vacuum chamber is an additional process step that is not typically a part of multilayer ceramic module fabrication processing sequences and the vacuum and thermal conditions that prevail during palladium deposition in an evaporator or sputtering machine pose the danger of decomposition and/or out-gassing of organic components of the green ceramic sheets, possibly leading to damage to the green ceramic sheets and to excessive contamination of the vacuum or sputtering systems. This patent is hereby incorporated by reference.
Journal of the Less Common Metals, 58 (1978) 61-74 discloses that on the basis of a grain boundary heterodiffusion model it is shown that Ni and Pd are the best sintering activators for Mo powder when present as coatings approximately 10 monolayers thick on the powdered surface.
IBM Technical Disclosure Bulletin, Vol. 12, No. 11, April 1970, page 1795 discloses that molybdenum can be used to metallize a high alumina green ceramic substrate.
IBM Technical Disclosure Bulletin, Vol. 16, No. 11, April 1974, page 3612 discloses forming a palladium coating on a conductor line on a multilayer ceramic material and then heating to partially diffuse the palladium into the conductor line.
IBM Technical Disclosure Bulletin, Vol. 17, No. 1, June 1974, pages 94-95 discloses placing a thin Mylar.RTM. film on a green ceramic sheet to improve its handling properties and dimensional stability, typically laminating the same at 150.degree. F., and 100-130 psi.
IBM Technical Disclosure Bulletin, Vol. 17, No. 8, January 1975, page 2280, discloses that green sheet instability after screening can be improved by the process which comprises punching locating holes in a suitable release film such as Mylar.RTM., screening a metal ground plane onto the release film using a mask, drying, transferring the ground plane pattern by aligning the screened pattern onto the release film and then laminating to the shiny surface of the green sheet, peeling off the release film, punching via holes, and screening metal paste into the via holes using a mask.
IBM Technical Disclosure Bulletin, Vol. 17, No. 8, January 1975, page 2331 discloses that the usual practice when soldering Mo is to plate a surface with an intermediate metal such as Pd before soldering.
IBM Technical Disclosure Bulletin, Vol. 19, No. 3, August 1976, page 222 discloses that molybdenum surfaces are conventionally activated for electroless nickel plating by a dip in a palladium chloride solution.
IBM Technical Disclosure Bulletin, Vol. 19, No. 10, March 1977 discloses that high density sintered molybdenum can be used for internal circuit patterns and for the top and bottom side metallurgy of multilayer ceramic modules. The molybdenum metallurgy is protected during sintering by forming a corrosion-resistant nickel-molybdenum layer.
IBM Technical Disclosure Bulletin, Vol. 21, No. 12, May 1979, page 4825 discloses that doctor bladed ceramic green sheets have a high and variable surface porosity. To overcome this problem, a resin coating is applied to the green sheet by coating a plastic film, typically a Mylar.RTM. sheet, and then transferring the coating to the green sheet with a low pressure lamination and peeling the sheet off. When the ceramic green sheet comprises polyvinyl butyral, a thin coating of the same organic material is applied to the Mylar.RTM. after the green sheet has been cast.